In wireless communications a random access procedure may be used to assign dedicated radio resources to a wireless transmit/receive unit (WTRU). A dedicated radio resource may be, for example, a specific preamble or physical random access channel (PRACH) resource. The random access procedure may be either contention-free or contention-based, and may include the following features: preamble transmission; random access response (RAR) reception, in which the RAR contains a grant for an uplink (UL) transmission; transmission of message3 (msg3) for contention-based random access; and contention resolution for contention-based random access (for example, the WTRU may determine whether the random access (RA) procedure was successfully completed).
FIG. 1 shows an example of a random access procedure including a WTRU and an evolved Node-B (eNB). The WTRU transmits a random access preamble. Then, the eNB responds with a RAR. Next, the WTRU may transmit a scheduled transmission, for example, a msg3 transmission. Then, the eNB may perform contention resolution for contention-based random access and the WTRU may determine whether the random access procedure was successful.
A WTRU initiates a random access procedure when one of the following events occurs: initial access to the network when the WTRU has no established connection, that is from the radio resource control (RRC)_IDLE state; a RRC connection re-establishment procedure, random access-physical downlink control channel (RA-PDCCH) order; handover; downlink (DL) data arrival during RRC_CONNECTED state requiring a random access procedure; or UL data arrival during RRC_CONNECTED state requiring a random access procedure, known as a random access scheduling request, (RA-SR).
In some wireless communications systems, such as, for example, Long Term Evolution (LTE), a WTRU may initiate a random access procedure due to one of the aforementioned events and a set of random access channel (RACH) resources is assumed to be available. The set of RACH resources is defined by a single index, prach-ConfigIndex, which may take a value between 0 and 63 and identify a preamble format to be used, as well as the set of subframes in which a preamble may be sent. The set of subframes may be further restricted by a provided physical random access channel (PRACH) mask index. In some systems, including orthogonal frequency division multiple access (OFDMA) wireless communication systems, the WTRU operates in a single UL carrier. Thus, there is no ambiguity as to which UL carrier may be used.
FIG. 2 shows a communication system 200 using a single UL carrier. The WTRU 201 communicates with the network 203 using a single DL carrier 205. A PRACH configuration is broadcasted on the DL carrier 205. The WTRU sends a preamble using available PRACH resources provided by the single UL carrier 207.
The WTRU selects a RACH resource within the available set. This involves the selection of a random access preamble followed by the determination of the next subframe containing an available RACH resource. The preamble is then transmitted in the next subframe (in the case of frequency division duplex (FDD)). In the case of time division duplex (TDD), the preamble is transmitted in a randomly selected RACH resource within the next subframe or the two subsequent subframes.
If a WTRU is configured to operate with multiple UL and DL carriers in a connect mode, multiple RACH resources are available for use to initiate a random access procedure, if required. It may be beneficial to define rules under which a WTRU may determine a RACH resource to utilize and may determine which sets of RACH resources may be considered available among the configured carriers.